The present invention relates to a null and gain adjustment mechanism for individually adjusting the rotary servo valves of a pair of interconnected hydrostatic pumps for zero displacement and for adjusting the fluid output of the pumps so that motors driven thereby are driven at the same rotational speed.
Hydrostatic transmissions are being employed in a variety of vehicles, including track-type vehicles. Such vehicles commonly employ separate hydraulic motors for each track with the hydrostatic transmissions including a pair of variable displacement hydraulic pumps, each of which supplies pressurized fluid to a respective one of the motors. Forward travel of the vehicle is accomplished by varying the displacement of the pumps. The displacement is varied by tilting the swash plates of the pumps in one direction to supply fluid to one side of the hydraulic motors while reverse motion is accomplished by tilting the swash plates over center in the opposite direction for supplying pressurized fluid to the other side of the hydraulic motors. The vehicle speed is determined by the angle of tilt of the swash plates. Steering of the vehicles is normally accomplished by decreasing or increasing the fluid flow to one of the hydraulic motors relative to the other.
One of the problems encountered with such systems is that of providing a mechanism having provision for precise synchronism between the drive motors. The need for synchronism is especially critical when the steering control is in a neutral position for travel of the vehicle in a straight line. Without precise adjustment or position correspondence of the pumps, the vehicle will not travel in a straight line, but will require continuous manipulation of the steering controls by the operator.
One approach to this problem is disclosed in U.S. Pat. No. 3,772,964 issued Nov. 20, 1973, and assigned to the assignee hereof.